linux-kernel - Re: [PATCH 2/5] pata_sis: Implement MWDMA for the UDMA 133 capable chips

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Date:	Tue, 17 Nov 2009 18:21:31 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Bartlomiej Zolnierkiewicz <bzolnier@...il.com>
Cc:	Alan Cox <alan@...ux.intel.com>, linux-kernel@...r.kernel.org,
	linux-ide@...r.kernel.org
Subject: Re: [PATCH 2/5] pata_sis: Implement MWDMA for the UDMA 133 capable
 chips

> > I'm not convinced because for the majority of drivers the libata timing
> > interface handles it. SiS needs it just because it does things by
> > precomputed tables. It's a one off interface.
> 
> Controllers based on *Intel* PIIX are in the disagreement with the above

No the PIIX is quite different. You use the matching PIO timing (which is
a short lookup and shorter code than even a helper function call). You do
not use the clipping instead you set bit 3 to ensure that PIO cycles
occur at low speed but MWDMA runs at the right speed. That is usually a
win over picking a lower mode as the PIIX can do ATAPI DMA happily.

So the only thing you can "share" is what would be a 4 byte table.

> paragraph and having generic helper to do conversion (without a clipping)
> would bring us a little step closer to killing a needless code duplication
> currently present in their ->set_piomode and ->set_dmamode methods.
> 
> [ In case somebody wonders: no, 'old' drivers don't have such duplication
>   and 'whoever did it didn't port it across' cause said 'whoever'-s were not
>   into the development of the square wheels.... ]

There is no duplication in the old drivers because they don't implement
the needed check and clipping/bit setting that I can see ;)

Alan
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